The invention relates to porous membranes comprising organopolysiloxane copolymers and their preparation, and also to their use for separating mixtures.
Separating mixtures using membranes is a long-established process. The separation of mixtures with membranes is usually accomplished with greater energy efficiency than by conventional separating methods, such as fractional distillation or chemical adsorption, for example. The search for new membranes with a longer lifetime, improved selectivities, better mechanical properties, a higher flow rate, and low costs, are aspects which are paid much attention in this search, in current membrane research.
Porous membranes of asymmetrical construction for separating any of a very wide variety of mixtures are known in the literature. Thus U.S. Pat. No. 3,133,137, U.S. Pat. No. 3,133,132, and U.S. Pat. No. 4,744,807 describe the preparation and use of asymmetrically constructed cellulose acetate membranes which are prepared by the phase inversion process. The process is likewise termed the Loeb-Sourirajan process. Membranes fabricated in this way have a porous under-structure and a selective layer. The thin outer layer is responsible for the separation performance, while the porous understructure leads to mechanical stability of the membranes. This kind of membranes is used in reverse osmosis plants for obtaining drinking water or ultrapure water from seawater or brackish water. Other membranes with this porous asymmetric construction are likewise known. Thus specifications U.S. Pat. No. 3,615,024, DE3936997, U.S. Pat. No. 5,290,448, and DE2318346 describe membranes comprising polysulfone, polyetherketones, polyacrylonitrile, and polyimide. Depending on the mode of preparation, very porous or more compact membranes are obtained. Typical applications of the membranes in these cases are reverse osmosis, ultrafiltration, nanofiltration, microfiltration, pervaporation, and the separation of gases. Through the polymers used, the polymers are partly hydrophilic. A consequence of this is that organic solutions are virtually impossible to separate, owing to the poor wetting.
The use of silicones as membrane material is likewise prior art. Silicones are rubberlike polymers having a low glass transition point (Tg<−50° C.) and a high fraction of free volume in the polymer structure. GB1536432 and U.S. Pat. No. 5,733,663 describe the preparation of membranes on the basis of silicones. Applications described include not only pervaporation but also the separation of gases.
Very thin silicone membranes, which would actually be necessary for optimum membrane performance, are impossible to handle, owing to the inadequate mechanical properties. In order to obtain the necessary mechanical stability of the silicones, the membranes described are always composite systems with a multilayer construction which is in some cases very complex and involved. The separation-selective silicone layer is always applied to a porous support substrate by methods such as, for example, spraying or solution application. Crosslinking takes place usually through a further step—for example, by aftercrosslinking with electromagnetic radiation or by the addition of catalysts.
A further application of silicones lies in the closing of defects in membranes that are used for separating gases. The polysulfone-based membranes described in U.S. Pat. No. 4,484,935 are sealed by an additional layer of silicone, in order to close small defects. The dense and compact silicone layer described therein is crosslinked by thermal treatment.
The use of organopolysiloxane copolymers as membranes is also prior art. US2004/254325 and DE10326575, for example, claim the preparation and use of thermoplastically processable organopolysiloxane/polyurea copolymers. The membrane applications for which the claimed silicones can be used are not described therein. Nor is the preparation of porous membranes described. Moreover, the use as membrane for separating gas/liquid, gas/solid, liquid/liquid, solid/liquid or solid/solid mixtures is not referred to in the patent specification. In addition, JP6277438 claims silicone-polyimide copolymer too as a material for preparing compact membranes. The applications recited therein are aimed at the separation of gases.
Likewise known in the literature are porous membranes comprising silicone-carbonate copolymers (JP55225703) and comprising silicone-polyimide copolymers (JP2008/86903). With both copolymers, however, the mechanical strength and the selectivity are not sufficient for technical deployment. With both copolymers, furthermore, there are virtually no physical interactions present, and this greatly lessens the thermal stability of the porous membrane structure. The silicone copolymers described, moreover, are very brittle, and this significantly hinders the preparation of typical wound membrane modules.
It is known, furthermore, that with silicone-carbonate copolymers the carbonate fraction in the copolymer must be high in order to obtain useful film-forming properties. Consequently, the favorable permeabilities of silicone are greatly impaired by the significantly less permeable polycarbonate.
A feature of the synthesis of silicone-imide copolymers is that the imidizing step must be carried out at temperatures of well above 250° C., and this is technically involved and makes the copolymers prepared expensive. Polyimides, moreover, have significantly poorer solubility, and this is unfavorable for the preparation of porous membranes.
This greatly restricts the use of both systems. Furthermore, for both copolymers, preparation is a very involved process, and this is unfavorable for industrial implementation.
In principle, the only polymers suitable for preparing porous membranes are those which possess sufficient mechanical strength and adequate flexibility. Furthermore, if preparation is carried out by means of the phase inversion process, the polymers must be soluble in an appropriate solvent which is miscible with the medium of the inversion bath. Typical polymers which can be processed in this way include cellulose acetate, polysulfones, polyvinylidene fluorides, polyetherimides, and aromatic polyamides.
The properties of normal silicones mean that they cannot be processed by means of the phase inversion process. Silicone membranes are prepared, in all of the processes described, by a multistage, involved, and expensive process. In addition, the preparation of very thin, compact separating layers on the basis of silicones is extremely difficult to accomplish technically. The preparation of porous separation-selective silicone layers is not possible with the methods described in the literature.
McGrath et al. in Advances in Polymer Science, 1988, Vol. 86, pp. 1-70 describe a series of different organopolysiloxane/polyurea/polyurethane/polyamide/polyoxalyldiamine copolymers.
Furthermore, Sava et al. in Revue Roumaine de Chimie, 2007, Vol. 52, pp. 127-133 describe the preparation of silicone-polyamide copolymers.